This IpAddress-typed field is described as the network mask for the static translation, noting that for one-to-one static NAT the mask should be zero. An admin uses a non-zero mask here only when the static entry is meant to translate a whole subnet rather than a single host, so getting this value wrong changes whether one address or an entire range gets remapped. For example, an admin doing a single-host publish would leave this at 0.0.0.0, while an admin remapping a /28 block of internal addresses would enter 255.255.255.240.